Effects of additives on perovskite formation in sol–gel derived lead magnesium niobate

The effects of ionic compounds (PbO, BaTiO3, PbTiO3, MgO) and covalent compounds (SiO2, GeO2, and B2O3), individually or as mixtures, on perovskite formation in sol–gel derived lead magnesium niobate, PbMg0.33Nb0.67O3 (PMN) were studied. The ionic additives were coprecipitated with the PMN while the covalent compounds were added as fine particulates. At a level of 10 mol% additive per mol of PMN, additions of PbO, BaTiO3, PbTiO3, as alkoxide solutions, increased perovskite formation from 82% to 90, 91 and 87% respectively. A combination of PbO (10–15 mol%) and BaTiO3 (8–9 mol%) increased perovskite formation further to 98%. The addition of 9 mol% MgO alone (also as an alkoxide solution) decreased perovskite formation to 77%, but a combination of PbO (10 mol%) and MgO (5 mol%) increased perovskite formation to 100%. The chemical effects of ionic compounds can be partially explained via the Goldschmidt tolerance factor. Compositions with high tolerance factors (>0.9) exhibited high (>90%) perovskite formation. Additions of 5 mol% GeO2, SiO2, and H3BO3 to modified PMN (14 mol% PbO and 9 mol% BaTiO3), resulted in significant decreases in perovskite formation, from 98% to 86, 80, 50%, respectively, when heated to 980°C for 8 h. The reduction in perovskite for undoped PMN, however, was nearly double (62% perovskite) when 5 mol% SiO2 was added.